Allow to override environment on spring client startup

[mildred]

Spring.env_override can be set to

• false (default) for current behaviour
• true to always override client provided environment variables in the application
• an array listing the environment variables to override in the application

The after_fork callback can take an ptional argument that contains the Spring provided environment to help the application detect when there is a mismatch between application environment and provided environment.

Fixes #653 and I believe will help with #420 (although a server restart is not performed, only environment overrides)